Diabetes is caused by metabolic abnormality mainly of glycometabolism, resulting from insufficient insulin secretion, decreased sensitivity of target cells of insulin and so forth, and principally characterized by noticeable hyperglycemia. If the hyperglycemia continues for a long period of time, serious complications arise in various organs and nerves such as retinopathy, nephropathy and neuropathy, which are caused mainly by vascular lesion. Therefore, for the treatment of diabetes, it is extremely important to control and maintain blood glucose level at a normal level, and methods for that purpose have been studied since old days.
For a type of diabetes where onset is gradual and insulin therapy is not necessarily required for life support (non-insulin dependent diabetes: NIDDM), blood glucose level can be controlled by combination of exercise therapy and drug therapy. As the drugs, insulin secretion promoters, one of orally available hypoglycemic agents, have widely been used clinically. However, since currently available insulin secretion promoters all promote insulin secretion non-dependently on glucose level, they cause problems of severe hypoglycemia or insufficient control of blood glucose if doses are not appropriate, and are not fully satisfactory drugs. If a hypoglycemic agent can be provided that is capable of promoting insulin secretion dependently on a blood glucose level, the agent is expected to be extremely useful for blood glucose control of patients suffering from diabetes because the risk of hypoglycemia due to an excess dosage can be avoided.
As condensed purine derivatives, Japanese Patent Unexamined Publication (Kokai) No. 3-204880, Journal of Medicinal Chemistry (J. Med. Chem.), 35, p. 3578, 1992, Journal of Medicinal Chemistry (J. Med. Chem.), 36, 2508, 1993, International Patent Publications WO98/15555 and WO 00/01388 disclose that the compounds represented by the following formula (A) have diuretic action, mild antiasthmatic action, antidemential action, bronchodilatation action, antiallergic action, antiulcer action, or hypoglycemic action. wherein R1A represents a hydrogen atom, a lower alkyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted aryl group, or a substituted or unsubstituted aromatic heterocyclic group, R2A represents a hydrogen atom, a lower alkyl group, a substituted or unsubstituted aryl group, or a substituted or unsubstituted aromatic heterocyclic group, R3A represents a hydrogen atom, a lower alkyl group, or a substituted or unsubstituted aralkyl group, V1A and V2A may be the same or different and each represents a hydrogen atom, a lower alkyl group, a substituted or unsubstituted aralkyl group, or a substituted or unsubstituted aryl group, and m represents an integer of from 0 to 3.
Journal of Medicinal Chemistry (J. Med. Chem.), 23, 1188, 1980 discloses that the compound represented by the following formula (B) has mild bronchodilatation action. 
Journal of Medicinal Chemistry (J. Med. Chem.), 40, 3248, 1997 and Japanese Patent Unexamined Publication No. 10-158267 disclose that the compounds represented by the following formula (C) have type IV phosphodiesterase inhibitory action (bronchodilatation action). wherein R1C, R3C and R4C may be the same or different and each represents a hydrogen atom or a C1–C6 alkyl group which may be substituted with a lower alkyloxy group or an acyl group, and p represents an integer of from 1 to 4.
Furthermore, EP390111A discloses that the compounds represented by the following formula (D) have adenosine antagonizing action. wherein R4D represents a hydrogen atom, a phenyl group, or β-D-ribofuranosyl group, WD represents a hydrogen atom, an alkyl group having 1 to 4 carbon atoms or an alkoxy group having 1 to 4 carbon atoms, V1D represents an aralkyl group, V2D represents a hydrogen atom or a phenyl group, and when V2D is a phenyl group, V1D may represent an alkyl group having 1 to 6 carbon atoms.